type of fumigant used to kill rats in enclosed spaces, such as holds of ships, boxcars, etc., 1926, from German Zyklon, commercial name of a type of hydrogen cyanide, said to be of unknown etymology, but it is the usual German form of the word cyclone. There were at least three varieties, A, B, and C, Zyklon-B being the one notoriously used in the Nazi death camps.